The RISC-V implementation of `cycleclock::Now` uses the user-space
`rdcycle` instruction to query how many cycles have happened since the
core started.

The only complexity here is on 32-bit RISC-V, where `rdcycle` can only
read the lower 32 bits of the 64-bit hardware counter. In this case,
`rdcycleh` reads the higher 32 bits of the counter. We match the powerpc
implementation to detect and correct for overflow in the high bits.